"We are going to appoint Mad Dog Mattis as our Secretary of Defense," he said. "But we're not announcing it till Monday, so don't tell anyone — Mad Dog. He's great. He is great." And on January 20th, mere hours following Trump's inauguration, the Senate confirmed Mattis's nomination.

Here are just a few things you should know about Trump's latest Cabinet pick.

1. He served in the Marine Corps for four decades before retiring in 2013.

NBC reports that Mattis is perhaps best known for leading the 1st Marine Division's "rapid drive into Baghdad" during the 2003 U.S. invasion of Iraq. He served as the 11th commander of the U.S. Central Command from 2010 to 2013, and previously commanded the U.S. Joint Forces Command while concurrently serving as NATO's Supreme Allied Commander Transformation. During the Iraq War, along with the 1st Marine Division, Mattis also commanded the I Marine Expeditionary Force and U.S. Marine Forces Central Command.

There's actually a technicality here, because Mattis is required to have been retired for at least seven years before taking the job — but he'll easily get that waived, especially given his impressive military résumé.

2. He considered running for president as an independent.

Mattis was courted by conservatives wanting to counter Trump's ascent during the general election and reportedly gave a presidential bid serious consideration before ultimately deciding not to run.

3. He has admitted he enjoys "brawling" with the enemy of war and once said, "It's fun to shoot some people."

"Actually, it's a lot of fun to fight," he said during a 2005 forum about war strategies in San Diego. "You know, it's a hell of a hoot. It's fun to shoot some people. I'll be right up-front with you, I like brawling. You go into Afghanistan, you got guys who slap women around for five years because they didn't wear a veil. You know, guys like that ain't got no manhood left anyway. So it's a hell of a lot of fun to shoot them"

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Another time, he told a group of Iraqi commanders, "The first time you blow someone away is not an insignificant event. That said, there are some assholes in the world that just need to be shot."

4. His nickname is "Mad Dog Mattis."

He's earned the nickname in part due to his straight-talking personality, "combative nature," and the fact that he openly discusses the brutality of war — for better or worse. However, he's reportedly not a huge fan of the nickname, which NBC reports is likely because it suggests he's far less strategy-focused and cerebral than he really is.

5. He's also known as the "Warrior Monk" because of his love for military strategy.

Mattis has essentially dedicated his whole life to the army, and according to ABC, "is known for a singular focus on military history, tactics and strategy, traits that, along with his unmarried status, led to his Warrior Monk nickname." Mattis has an extensive library and is himself a writer, and has always valued the importance of strategy over all else.

Multiple sources report that he has famously told his Marines, "The most important six inches on the battlefield is between your ears." In a letter to the 1st Marine Division, he wrote, "You are part of the world's most feared and trusted force. Engage your brain before you engage your weapon."

6. He's a favored pick for Democrats and Republicans alike.

In an interview with NPR, Michèle Flournoy — who served in President Barack Obama's Defense Department and was rumored to be the top pick for Secretary of Defense if Hillary Clinton had won the election — praised Mattis as a wonderful choice for the role.

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"General Mattis is a storied and much respected military leader," she said. He's a student of history. He's a strategic thinker, and he also has real passion for, you know, the care of the men and women in the U.S. military and their families. So, you know, I think he would be an outstanding candidate."

7. He takes a strong stance on Iran.

During his time as the 11th commander of the U.S. Central Command, Mattis butted heads with President Barack Obama over his Iran policy and was extremely vocal about his disapproval over the president's decision to agree to the Iranian nuclear deal.

"The Iranian regime, in my mind, is the single most enduring threat to stability and peace in the Middle East," he said during an April Washington think tank. "Among all the issues facing us in the Middle East, I think Iran is actually foremost. And yet at the same time, it appears here in Washington that we’ve forgotten how to keep certain issues foremost."

8. He's currently a fellow at Stanford University.

Mattis is Stanford University's Davies Family Distinguished Visiting Fellow at the Hoover Institution, as part of the "Military History/Contemporary Conflict Working Group" research team. He is currently studying national security issues with a focus on "strategy, innovation, the effective use of military force, and the Middle East," while also working on a book about leadership.

9. He co-wrote the Counterinsurgency Field Manuel with Army Gen. David Petraeus.

As Slate reports, Mattis and Petraeus pushed for the creation of the Army/Marine Counterinsurgency Field Manuel in 2006 as a guide for troops serving in Afghanistan and Iraq. According to the U.S. Government Counterinsurgency Guide, "Counterinsurgency (COIN) is the blend of comprehensive civilian and military efforts designed to simultaneously contain insurgency and address its root causes. Unlike conventional warfare, non-military means are often the most effective elements, with military forces playing an enabling role." Essentially, the field manual encourages using less force in the short-term at a higher risk in order to establish longer-term relations with civilians that could benefit military efforts in the long run.

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10. He has reportedly clashed with Trump's transition team.

According to the Washington Post, Mattis and Trump's transition team have clashed over candidates for the top roles in the Department of Defense. Trump's team reportedly had people in mind for those positions and are not open to the idea of "Never Trump" Republicans, but Mattis thought he would have significant influence in the selections. Mattis was not happy that Trump announced Vincent Viola as secretary of the army without his input, CNN reported. Incoming White House press secretary Sean Spicer responded to the Washington Post story, tweeting, "Great transition at DoD. Reports to contrary completely false and come from sources who do not have any knowledge of our transition efforts."